HI
Cornwall's planning laws are overseen by Cornwall Council http://www.cornwall.gov.uk/ My advice would be to call them first but, also speak to a couple of local architects, architectural technicians and/ or planning experts who have experience in this field (no pun intended). Once you have established where and hat you are likely to be able to build, please feel free to give us a call for any more help you may need. On a personal note, I would like to see a lot more imaginatively designed Eco-homes in our area. Good luck :)
Chris